A. Field of the Invention
The invention relates generally to light emitting devices, more particularly, to semiconductor optical lasers having an improved design to reduce internal optical loss.
B. Description of the Related Art
Semiconductor lasers are attractive sources for optical power generation because they are more efficient, smaller, and less expensive than other types of lasers. There are increasing applications for higher output power single-spatial-mode semiconductor lasers such as for pumping optical fiber amplifiers/lasers, optical wireless communications, optical fiber transmitters, and other laser applications. The output optical power from a semiconductor laser (P) is governed by a simple relationship: P=Sxc3x97(Ixe2x88x92Ith). As the current (I) injected into the semiconductor laser is increased above the threshold current value (Ith), the output power increases proportionally with a constant of proportionality S known as the slope efficiency. High power semiconductor lasers are operated at current injection levels high above threshold. Therefore, the slope efficiency is the most dominant parameter determining the attainable maximum output power. It is at least partially for this reason that increasing the slope efficiency is critical for increasing the output power of semiconductor lasers. An additional benefit of increased slope efficiency is an increase in the net electrical-to-optical conversion efficiency which reduces power consumption and heat generation. However, for single-mode applications, such as coupling light into optical fibers, it is crucial that the angular intensity profile of the optical radiation emitted by the semiconductor laser remain nearly diffraction limited. This requires that the optical mode at the laser output facet must be single-lobed and preferably have an intensity profile that is substantially gaussian. Therefore, increasing the slope efficiency of a single-mode laser is most beneficial if the optical mode at the output facet retains a single-lobed intensity profile.
The slope efficiency is directly influenced by the background optical loss that the optical mode experiences inside the semiconductor laser. A lower internal optical loss results in a higher slope efficiency. For conventional semiconductor lasers, more than 50% of the lasing mode is propagating in the doped cladding region, where the optical loss due to free-carrier absorption is significant. This loss is particularly large for the p-side cladding. The internal loss can be reduced by using broadened waveguide structures as described in Electron. Lett., vol. 32, pp. 1717-1719, 1996. However, a broadened waveguide may cause adverse effects as well. The increased carrier transport time in the broadened guiding layer will increase the population of carriers in the guiding layers and enhance the carrier recombination. Consequently, the temperature sensitivity of the slope efficiency is deteriorated at high temperatures. The modulation bandwidth will also be degraded for broadened waveguide lasers since the carrier transit time is increased.
Higher slope efficiency and reduced internal loss can also be achieved by using low doping concentrations in the cladding layer as suggested by J. W. Pan, et. al. at the 10th Int. Conf. on Molecular Beam Epitaxy, Cannes, France, 1998. However, electron leakage current over the low doped cladding layer has been found to become significant with temperature, and render threshold current and slope efficiency highly temperature sensitive.
Another approach to reduce the internal loss employs an asymmetric transversal layer design to reduce the confinement factor of the optical mode in the active region [IEEE Photon. Technol. Letters, vol. 11, no. 2, pp. 161-163, Feb. 1999]. The confinement factor is the ratio of the portion of optical mode that overlaps with the active region to the entire optical mode. This device uses an xe2x80x9coptical trapxe2x80x9d layer on the n-side of the active region to lower the confinement factor. A fundamental disadvantage of this approach, however, is that the transverse optical mode is no longer a single lobed gaussian shape. Instead, it has two distinct lobes. Thus the far field emission pattern from the laser is no longer diffraction limited. This distortion of the far field pattern significantly degrades device performance including reduced coupling efficiency of the laser to a single mode fiber.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above and other problems with the prior art.
According to one aspect of the present invention, an optical device is provided comprising a gain section adapted to emit radiation at a radiation wavelength, a coupling section adjacent to the gain section for transitioning radiation between an active waveguide and a passive waveguide, and a passive section adjacent to the coupling section supporting a single-lobed optical mode in the passive waveguide at the radiation wavelength. The passive waveguide has an index of refraction and dimension such that the confinement of the radiation within the active waveguide in the gain section is reduced.
According to another aspect of the present invention, a method of generating optical radiation is provided comprising the steps of generating a multi-lobed lasing mode in a gain section, one lobe of the lasing mode being substantially in an active waveguide in the gain section, and coupling the multi-lobed lasing mode to a single lobed optical mode substantially in a passive waveguide. The passive waveguide has an index of refraction and dimension such that the confinement of the multi-lobed lasing mode within the active waveguide in the gain section is reduced. Preferably, the passive waveguide has an index of refraction and dimension such that the passive waveguide supports a single lobed optical mode in the passive section. More preferably, the passive waveguide has an index of refraction and dimension such that the single lobed optical mode is a substantially diffraction-limited gaussian mode.
According to another aspect of the present invention, an optical device is provided comprising a gain section adapted to emit radiation at a radiation wavelength within an active waveguide, the gain section adapted for supporting a multi-lobed optical mode comprising a first and second lobe at the radiation wavelength, a coupling section adjacent to the gain section for transitioning the radiation from the active waveguide to an n-doped passive waveguide, and a passive section adjacent to the coupling section for supporting a single-lobed optical mode in the n-doped passive waveguide. The passive waveguide has an index of refraction and dimension such that confinement of the multi-lobed optical mode at the radiation wavelength within the active waveguide in the gain section is reduced. The peak intensity of the first lobe in the gain section occurs within the active waveguide and the peak intensity of the second lobe in the gain section occurs within the passive waveguide.